The invention relates to a process for producing a catalytically active material, in which titanium dioxide together with an oxide or a dissolved compound of tungsten and/or vanadium is processed to form a shapeable material. The shapeable material is dried to form a shaped body and the shaped body is calcined. The invention also relates to a catalytic converter having a catalytically active material produced in this manner and to using the catalytic converter for breaking down dioxins and/or furans.
A process of this type for the production of a catalytically active material, and a catalytic converter of this type are known, for example, from Published German application DE 24 58 888 A1. Using titanium dioxide-containing catalytic converters to break down dioxins and/or furans is also known from European Patent EP 0 471 033 B1, European Patent EP 0 252 521 A1 or International Publication WO 91/04780.
As has been extensively outlined in the prior art mentioned above, to produce the catalytically active material, the titanium dioxide is initially processed, with the addition of a solvent containing tungsten oxide and/or vanadium oxide, to form a shapeable material. Alternatively, the shapeable material can be produced by adding titanium dioxide to a dissolved compound of tungsten and/or vanadium. By way of example, ammonium paratungstate or vanadyl sulfate are used as dissolved compounds of this type. The shapeability of the material is influenced by the solvent content.
Naturally, additives, such as for example, ceramic fibers, glass fibers and/or film-forming agents, may be admixed with the shapeable material.
Then, the shapeable material is, by way of example, extruded to form a honeycomb body with passages through which a medium can flow. The shapeable material can alternatively be processed further to form pellets, or can be applied as a coating to a supporting body. The coating of the supporting body is effected by rolling the material on or by immersion. The extruded or processed material is then dried and is then referred to as a xe2x80x9cshaped bodyxe2x80x9d. The shaped body is then calcined to produce the catalytic activity.
Solid bodies consisting of the catalytically active material which has been produced in this manner, or support bodies which have been coated with this material, are in widespread use as catalytic converters for breaking down nitrogen oxides using the SCR (Selective Catalytic Reduction) process or for breaking down dioxins and/or furans. In the SCR process, i.e. the selective catalytic reduction process, nitrogen oxides are reacted in the presence of a reducing agent at the catalytically active material to form harmless nitrogen and water. Dioxins and/or furans are oxidized at the catalytically active material to form harmless compounds.
To break down dioxins and/or furans, which in this context is understood as meaning the polyhalogenated cyclic diethers and ethers, catalytic converters comprising the catalytically active material described above are used at temperatures of between 250 and 350xc2x0 C. Below 250xc2x0, however, the catalytic activity of the active material becomes insufficient to still achieve effective reduction in the level of halogenated hydrocarbons. However, low temperatures of this nature occur, for example, in the off-gas from a garbage incineration plant or a sintering plant. It is also known that at temperatures below 200xc2x0 C., dioxins and/or furans are formed again during cooling in the off-gas as a result of the DeNOVO synthesis in the presence of organic carbon compounds, alkali metal chlorides or alkaline-earth metal chlorides and metal compounds acting as catalysts. In this case too, it is necessary to treat the off-gas in a temperature range in which the catalytic activity of said catalytic converter is no longer sufficient.
It is accordingly an object of the invention to provide a process for producing a catalytically active material that has an improved catalytic activity compared to prior art catalytically active materials of this type, and that, in particular, has an improved catalytic activity at temperatures below 250xc2x0 C. A further object of the invention is to describe a catalytic converter of the type described in the introduction which enables pollutants, and in particular dioxins and/or furans, to be broken down effectively even at low temperatures. A further object of the invention is to describe a use of a catalytic converter of this type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a process for producing a catalytically active material, that includes steps of: processing titanium dioxide together with a component selected from the group consisting of an oxide of tungsten, a dissolved compound of tungsten, an oxide of vanadium, and a dissolved compound of vanadium to form a shapeable material; drying the shapeable material to form a shaped body; calcining the shaped body; impregnating the shaped body with a solution containing titanium and vanadium; and drying the shaped body that has been impregnated.
The object of the invention is achieved by the fact that, in a process of the type described in the introduction, the shaped body is additionally impregnated with a dissolved compound of titanium and vanadium and is dried again.
Extensive tests have demonstrated that a catalytically active material which has undergone further treatment in this manner has an excellent catalytic activity, in particular for the oxidation of dioxins and/or furans, in a temperature range of below 250xc2x0 C. At temperatures of over 250xc2x0 C., a catalytically active material which has undergone further treatment of this type also has an improved catalytic activity compared to catalytically active materials which have not undergone further treatment of this type.
It is postulated that the subsequent impregnation with a titanium- and vanadium-containing solution and the subsequent drying leads to the formation of an extremely active, hitherto unrecorded vanadyl-titanyl compound. The shaped body is advantageously impregnated after the calcining. Although in principle the shaped body which has not yet been calcined can also be impregnated with the solution, particular care is required, since the uncalcined shaped body easily breaks up in solution.
In accordance with an added feature of the invention, for catalytic activity, a solution with a molar ratio of titanium to vanadium of between 0.1 and 10 is used for the impregnation operation. The best results with regard to catalytic activity can be produced within this molar ratio.
In accordance with an additional feature of the invention, the shaped body is advantageously impregnated with a solution of vanadyl sulfate and titanyl sulfate. Both vanadyl sulfate and titanyl sulfate have an excellent solubility, in particular in water. Naturally, it is also possible to use a different compound, such as for example ammonium metavanadate, which is soluble in organic solvents instead of vanadyl sulfate. Titanic acids could be used instead of titanyl sulfate.
In accordance with another feature of the invention, tungsten is additionally dissolved in the solution. This can also have a beneficial effect on the catalytic activity of the catalytically active material.
Ammonium paratungstate which, like titanyl sulfate and vanadyl sulfate, is readily soluble in water is recommended as a soluble compound for tungsten. In addition, however, it is also possible to use ammonium heptatungstate, which can likewise be dissolved in hot water or a strong acid.
In accordance with a further feature of the invention, it is advantageous, both with regard to environmental aspects and with regard to cost, if the solution used is an aqueous solution. This makes it possible to dispense with expensive, possibly environmentally hazardous organic solvents and acids altogether during production. With regard to the catalytic activity, it is also advantageous if the further drying which takes place after the impregnation operation takes place at a temperature of between 250 and 400xc2x0 C. This temperature should in particular be selected in such a way that it is higher than the subsequent temperature at which the catalytically active material is used.
In accordance with a further added feature of the invention, the calcining takes place at a temperature of between 500 and 600xc2x0 C.
With the foregoing and other objects in view there is also provided, in accordance with the invention, a catalytic converter that includes a catalytically active material which has been produced in accordance with the invention.
With the foregoing and other objects in view there is also provided, in accordance with the invention, a method of using a catalytic converter of this type to break down dioxins and/or furans. In particular, the catalytic converter is recommended for use in the low-temperature range at temperatures of less than 250xc2x0 C.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a process for producing a catalytically active material, catalytic converter having a catalytically active material produced in this manner, and use of the catalytic converter to break down dioxins and/or furans, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.